NOx emissions from vehicles with internal combustion engines are an environmental problem recognized worldwide. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations. In conventional gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel powered vehicles and vehicles with lean-burn gasoline engines, however, the exhaust is too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions from diesel powered vehicles and lean-burn gasoline engines. One set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NOx catalysts, NOx adsorber-catalysts, and selective catalytic reduction (SCR). These methods are promising, but have associated costs in terms of equipment and often a fuel penalty. Complementary approaches focus on limiting NOx formation by the engine.
NOx formation can be limited by avoiding high combustion temperatures. Methods of avoiding high combustion temperatures include retarding the ignition timing, cooling intake air, and reducing the oxygen concentration through exhaust gas recirculation (EGR). Retarding the ignition timing makes combustion less efficient, and therefore has a fuel penalty. Cooling the intake air is helpful, but does not lower the temperature to the desired degree.
EGR can reduce NOx production. It is typically used selectively according to engine operation conditions (torque and speed). Generally speaking, there are two types of EGR, external EGR (sometimes referred to as cooled EGR) and internal EGR (sometimes referred to as non-cooled EGR). External EGR includes high pressure EGR and low pressure EGR. High pressure EGR involves recirculating exhaust drawn from a high pressure portion of the exhaust stream. Generally, a high pressure portion of the exhaust stream is a portion upstream of a turbine placed in the exhaust stream to drive an intake air turbo charger. Low pressure EGR involves drawing exhaust from a low pressure portion of the exhaust stream, such as a portion downstream of a turbine. Low pressure EGR requires a pump or compressor to pressurize the exhaust. External EGR systems normally include a heat exchanger to reduce the temperature of the recirculated exhaust to improve the volumetric efficiency of the engine. A heat exchanger is typically placed at the point from which the exhaust is drawn and typically uses engine coolant as a cooling medium.
U.S. Pat. No. 4,267,812 describes an EGR cooler for use in conjunction with an external EGR system. The cooler is said to be more compact than conventional EGR coolers, and is designed for mounting on an intake manifold with little or no modification to the intake manifold.
Internal EGR is achieved through valve timing. Internal EGR usually involves opening an exhaust valve during an intake stroke, whereby exhaust is drawn into the cylinder from the exhaust manifold. Internal EGR can also be achieved by early exhaust valve closing, which leaves a portion of the exhaust in the cylinder at the end of an exhaust stroke. A further method of achieving internal EGR is to open an intake valve during an exhaust stroke, whereby exhaust flows into the intake manifold, however, this approach is generally avoided because it results in heating of the intake manifold, which in turn heats the intake air, increasing its volume and disrupting the fuel-air ratio.
There continues to be a long felt need for better ways of limiting NOx emissions from diesel engines.